Microwave systems are used in a variety of fabrication processes. Often, microwaves are generated by a microwave source and are then delivered to an intended load. In the case of microwave casting, for example, the microwave power may be used to melt metals held in a crucible. To improve the efficiency of this process, the amount of power delivered to the intended load is maximized so that more of the microwave power is transferred to the load instead of being lost as heat, reflected back to the source, etc. One method for maximizing power delivery and reducing loss of microwave power is impedance matching. The end goal of impedance matching is to match the impedance of the intended load to the impedance of the source of power by placing an impedance-matching device between the source and the load.
A stub tuner is one type of impedance matching device. A stub tuner typically consists of one or more variable length stubs of transmission line or waveguide that are located between the source and the load. The operative length of each stub is increased or decreased in order to modify the impedance. In the case of a microwave system, stubs are positioned in “pockets” adjacent the waveguide between the microwave source and the intended load. The stubs are inserted into and extracted out from the waveguide, thereby varying their operative length, to adjust the impedance. One historic method for achieving this insertion and extraction is by attaching a threaded rod to the end of the stub and then inserting and retracting the stub by turning the threaded rod.
Some microwave applications require a high-vacuum environment to operate most effectively. For example, a high-vacuum environment is required for semi-conductor fabrication. In certain other cases, there is a need for a specific chamber environment in the microwave processing chamber, such as an inert environment. This requires creating a vacuum in the chamber and then backfilling the chamber with the desired chamber gas. Historically, the use of a stub tuner in connection with high vacuum or highly pressurized microwave applications has been difficult. A primary reason for this difficulty is that the structure of existing stub tuners does not allow for a vacuum or pressurized environment (hereinafter, collectively or interchangeably referred to as “pressurized” environments, unless specifically noted) to be created if the stub tuner is located within that environment. As mentioned above, threaded rods have been used to adjust the positioning of stubs in stub tuners. Such method and other existing methods for adjusting the stub length require some physical perforation of the pocket to connect the stub located within the pocket and the adjustment means located outside of the pocket for adjusting the amount of stub length inserted into the waveguide (the operative length).
Various methods have been used to allow the use of a stub tuner in a highly pressurized environment. One such method requires separating the microwave system into a pressurized section and a non-pressurized section separated by a gas-sealed barrier, such as a quartz window, that permits the transmission of microwaves through the barrier. The intended load that requires the pressurized environment is located in the pressurized section whereas the stub tuner is located on the other side of the gas-sealed barrier in the non-pressurized section. A common problem with this type of design, however, is that the gas-sealed barrier can become contaminated so that it begins coupling with the microwaves or otherwise interferes with the microwaves that reach the intended load after the microwaves have been tuned. Also, the gas-sealed barrier itself has an impedance that must be accounted for after the microwaves have been tuned. Many times, wave matching features and/or filters must be incorporated into the design of the microwave system to reduce these interferences and to account for the impedance of the gas-sealed barrier. Unfortunately, these added structures adversely affect the efficiency of the system because the best tuning for the window may not be the same as the best tuning for the pressured cavity. In other words, although the stub tuner may be optimized so that the maximum amount of power is transmitted through the gas-sealed barrier, that tuning may not be ideal for transmitting the maximum amount of power to the load in the processing chamber.
Accordingly, what is needed is a stub tuner configured to be in fluid communication with a microwave processing chamber so that tuned microwaves are directed to the chamber and load without being contaminated or obstructed.